The present invention relates to hot melt adhesives, and more particularly to a hot melt adhesive made from a blend of a polypropylene-based polymer, a polyolefin elastomer, and an amorphous polyolefin. These adhesives exhibit excellent molten and pre-set flow allowing them to wet out substrates yet develop properties needed to form and maintain strong bonds making them useful for hygiene, construction, and packaging applications.
Hot melt adhesives are used to bond substrates using a wide range of application methods and process conditions for a large variety of end-uses. For example, hot melt adhesives are employed to bond non-woven materials, polymeric films, and elastomeric components in numerous fabricated articles. Laminate structures with hot melt adhesives serving to bind nonwoven materials and elastomeric components in a form of strands, films, or any other continuous or discrete form are especially useful in hygiene products like diapers.
Processing of hot melt adhesives is linked to their ability to be melted, transported and/or coated in a molten stage at the final location where the bond is required. Molten adhesives can be sprayed or coated as thin layers. Once cooled, the adhesive needs to fulfill multiple requirements such as suitable bond strength measured by peel force or bond retention during or after mechanical stress, and during or after various thermal conditions.
Hot melt adhesives can be based on polymers such as polyolefins (ethylene- or propylene-based polymers), or functionalized polyolefins (ethylene or propene copolymers with oxygen containing monomers), or styrenic block copolymers containing at least one rubbery phase, like styrene-isoprene-styrene (SIS), or styrene-butadiene-styrene (SBS) polymers. Styrenic block copolymers are commonly employed due to their dual characteristics, i.e. cohesion of the styrenic phase associated with the rubbery behavior of the poly(butadiene) or poly(isoprene) phase.
Over the years, many different olefinic polymers have been used in the formulation of hot melt adhesives. The first of these were amorphous polypropylenes (APP) that are characterized by having a random steric orientation of the pendant methyl group along the carbon chain. The lack of stereoregularity frustrates APP systems from crystallizing making them soluble materials that could be combined with various tackifiers, plasticizers, waxes and fillers to produce a hot melt adhesive for a variety of end-use applications.
Later, olefin polymers became available that had much improved properties over the original amorphous polypropylene polymers. These are referred to as amorphous poly alpha olefins (APAO). They are generally produced using Ziegler-Natta catalysis and can be made using a variety of monomers, including but not limited to propylene, ethylene and butene. Various copolymers and terpolymers are produced by a number of manufacturers. They include Evonik Industries, who produce the Vestoplast® polymers; REXtac, LLC, who produces the Rextac® range of materials, and Eastman Chemical, manufacturers of the Eastoflex® line of polymers. They are all characterized by having a very low degree of crystallinity as measured by DSC. As commercially produced, they are random polymers having broad molecular weight distributions.
More recently, metallocene and single-site catalysis have been developed to produce polyolefins with more precisely tailored properties. For example, the molecular weight distribution can be controlled to provide polymers with significantly narrower polydispersity values compared to those produced employing traditional Ziegler-Natta catalysts. Metallocene and single-site catalysts also can be designed that display high comonomer incorporation rates compared to Ziegler-Natta catalysts. This allows high levels of comonomers, such as 1-butene and 1-octene, to be incorporated into ethylene-based copolymers and provide low density polyethylene copolymers. Examples of ethylene-based copolymers of this class include Affinity® and Engage® polymers from the Dow Chemical Company. Similarly, metallocene and single-site catalysts have been developed which allow propylene-based copolymers to be produced that contain high levels of ethylene and/or other alpha-olefins. Examples of propylene-based copolymer systems include Vistamaxx® polymers from ExxonMobil and Versify® grades available from the Dow Chemical Company. Metallocene and single-site catalysts also can also be exploited to control the chain architecture of polyolefins and their copolymers. These catalysts govern the degree of stereo- and regio-defects along the polymer chains and, in turn, the crystallinity and final properties. Control of polymer stereo-regularity using these catalysts can be performed such that pendant substituents of neighboring backbone carbons (“diads”) are primarily arrayed in an identical (“meso”) fashion to provide highly isotactic polymers. Conversely, metallocene and single-site catalysts can be designed such that side-branch alkyl groups are oriented in an opposing fashion to afford syndiotactic polymers. Recently, catalysts have been developed that target a fixed level of stereo-defects to allow for fine control of polymer properties. For example, a highly isotactic polypropylene homopolymers with very low levels of stereo-errors (racemic content less than 0.50 mol %) are generally stiff, high melting materials. Conversely, using catalysts designed to selectively introduce a controlled level of racemic errors can provide materials that, while chemically identical, display enhanced flexibility and are much lower melting. Examples of this class of polymers include L-MODU S400, S600, and S901 propylene-based polymers available from Idemitsu Chemicals. While these polymers have been used to make hot melt adhesives with better adhesion characteristics, they have not been widely used in applications requiring high flow, excellent wet out, and strong initial bonding that is maintained with long-term aging.